Magnetic pulse controlled microstructure development in Co49Ni21Ga30 single crystals

A pulsed magnetic field (MF) up to 0.6?T leads to the generation of fine microstructure through the change in the dynamics of the interaction process between the MF and the magnetic- and microstructure in Co₄₉Ni₂₁Ga₃₀ magnetic shape memory alloy single crystals. This paper addresses the impact of pulsed and permanent MF on the transformation behaviour in Co₄₉Ni₂₁Ga₃₀. A pulsed MF can result in different microstructural accommodation processes in stress-induced martensite, as compared to an application of a mechanical stress or permanent MF. In a pulsed MF, the development of new nano-twin systems in the martensitic structure of the CoNiGa alloy and the formation of fine magnetic domains at the microscale are observed.     

Effect of orientation on the high-temperature superelasticity in Co49Ni21Ga30 single crystals

The temperature interval ΔT _SE of superelasticity in [001]-, [$ \bar 1 $ \bar 1 23]-, and [$ \bar 1 $ \bar 1 24]-oriented Co₄₀Ni₂₁Ga₃₀ (at. %) single crystals strained at compression has been studied. It is established that ΔT _SE in the [001]-oriented single crystal amounts to 441 K and the reversible B2-L1₀ martensite transformations in loaded samples take place at T ₂ = 698 K. In [$ \bar 1 $ \bar 1 23]- and [$ \bar 1 $ \bar 1 24]-oriented samples, ΔT _SE decreases to 233 K and the superelasticity is observed up to T ₂ = 523 K.     

NiTiHf高温形状记忆合金的相变行为和形状记忆效应

利用透射电镜、X射线衍射仪和示差热分析仪系统分析了Ni49Ti36Hf15合金的相变行为及其形状记忆效应。结果表明，Ni49Ti36Hf15合金的热马氏体变体间构成典型的自协作组态，主要呈矛头状、镶嵌块状和楔状3种形态，亚结构主要为（001）复合孪晶。随着热循环次数的增加，相变温度降低，经50次热循环后，相变温度随热循环次数增加变化趋势不明显。固溶处理Ni49Ti36Hf15合金在20－184℃范围内弯曲变形时，呈现良好的形状记忆效应，其最大可恢复应变可达3％。形状恢复率随着弯曲变形温度的增加而下降，当弯曲变形温度大于317℃时，形状恢复率下降为0。     

Pd对多晶Ni56.5Fe17Ga26.5合金马氏体相变和力学性能的影响

通过电弧炉冶炼了在Ni56.5Fe17Ga26.5合金中添加Pd的合金试样,利用SEM,XRD,TEM,DSC,TG和万能材料试验机研究了Pd对Ni56.5Fe17Ga26.5合金马氏体相变和力学性能的影响.结果表明,添加Pd促使合金中形成较多的γ相,并且使其球化,提高了合金的塑性.通过DSC和TG测量了马氏体相变温度...     

铁磁形状记忆合金CoNiGa马氏体相变特征的研究

应用具有变温样品室的X射线衍射仪及扫描电镜研究了铁磁形状记忆合金Co50Ni20Ga32样品马氏体相变的特性.研究发现,多晶Co50Ni20Ga32合金具有可恢复的热弹性马氏体转变特性.y相的存在对马氏体转变的自协作形貌有很大影响.γ相含量多时,马氏体变体的自协作形貌以金刚石形为主,γ相含量少时,马氏体变体的自协作形貌以平行形为主.在大约520℃以上时,y相的含量随温度升高而增加.     

铁磁形状记忆合金Ni2MnGa多晶的磁-力学特性

对外加磁场、应力场共同作用下的铁磁形状记忆合金多晶的磁-力学特性进行了实验测试与研究,分别获得了两种组分Ni52Mn27Ga21和Ni54Mn25Ga21多晶样品在不同磁场倾角下、不同预加应力下的磁化曲线和磁滞回线;以及不同外加磁场及磁场倾角下的应力-应变曲线和磁致应变曲线等磁-力学特性曲线.结果表明铁磁形状记忆合金多晶沿样品轴向所测的饱和磁化强度随磁场倾角的增大而减小,施加预应力并不显著影响样品的磁化曲线和磁滞回线;各种角度时不同磁场对两种多晶样品的应力-应变关系影响均很小.     

影响Fe-Mn-Si-Cr-Ni形状记忆合金相变点的因素

研究了变形量和回复退火温度对Fe-14Mn-5Si-8Cr-4Ni形状记忆合金相变点的影响。结果表明：当回复退火温度为673K时，Af点和Ms点都随变形量的增加而显著增加，As点增加较缓慢；303K加热前和加热后合金电阻率之差△ρ也随变形量的增加而增加，可回复变形量随变形量的变化与△ρ的变化是一致的。Fe-Mn-Si-Cr-Ni形状记忆合金的形状回复来源于应力诱发γ→ε马氏体转变及其逆转变；当变形量为10%时，Ms点随回复退火温度的增加而显著下降。     

High undercooling effect on magnetic shape memory Co–Ni–Ga alloys

The martensitic microstructure and crystal structure evolution of Co₄₆Ni₂₇Ga₂₇ alloys under different undercooling condition was investigated. The increase of reverse martensitic transformation temperature was found with undercooling in 22 to 165 °C undercooling range, but decreased with further high undercooling. The reasons for these have been discussed using the elastic strain energy mechanism and grain refinement caused by internal stress during rapid solidification.     

Ni50.1Mn24.1Ga20.3Fe5.5形状记忆合金多晶纤维的双程形状记忆效应

通过熔体抽拉技术制备Ni_50.1Mn_24.1Ga_20.3Fe_5.5多晶纤维,采用步进式热处理释放因快速凝固引起的内应力和缺陷,利用场发射扫描电子显微镜、透射电子显微镜、XRD衍射仪对其微结构和相结构进行表征,采用动态机械拉伸仪测试其相变行为和双程形状记忆性能。结果表明:热处理后原子有序度显著提高,孪晶界平直,在恒应力作用下一个热循环中母相和马氏体相的形状得到完全恢复。双程形状记忆曲线显示了热弹性马氏体相变的两个基本特征:可逆性和热滞性。在热循环实验中,纤维被加载到198 MPa时,其马氏体态总应变达到1.32%。根据热机械拉伸测量,发现相变温度遵循Clausius-Clapeyron关系式。与诸如Ti-Ni和Cu-Al-Ni的其他合金相比,Fe掺杂的纤维显示出较小的应变-应力依赖性,在恒应变输出的驱动中是有益的。     

时效Ti49Ni51合金的显微组织与双程形状记忆效应

采用弯曲试验和透射电镜分析研究了时效Ｔi49Ni51合金的显微组织与双程形状记忆应用,结果表明,５００６℃,１h时效处理试样中,相呈透镜片状,周围有较强的应变场初度,而５００℃,３０h时效试样中,ＴiNi4相已聚集长大成粗片状,与基体失去共格,５００℃,１h时效处理试样在训练后可获得较好的双程形记忆效应,时效Ｔi49Ni51合金中ＴiNi4相粒子周围的共格应变场和主地位错交互作用可以产生有效内应力场,从而诱发双程形状记忆效应。     

On the anisotropic thermal conductivity of shape memory alloy single crystals

Summary This paper addresses the issue of anisotropy in overall thermal conductivity of a shape memory alloy microstructure that is single crystalline in the high temperature austenitic state. The Voigt and Reuss bounds of the thermal conductivity are given. It is shown that the trace of the Voigt bound is independent of the specific variants that may exist and their orientations in the microstructure, whereas it is not so for the Reuss bound. It is also shown that both bounds of a purely self-accomodated martensitic microstructure originating from a cubic parent phase are isotropic and independent of the geometric properties of the variants. The bounds are then applied to the study of orthorhombic martensitic SMAs that are single crystalline in the cubic austenitic state. Using the resolved shear stress approach for variant selection, the impact of anisotropy in the thermal conductivity components of the orthorhombic martensite is studied for a single variant microstructure where the Voigt and Reuss bounds are used as estimates for the thermal conductivity of a cylindrical domain. Their impact on the normalized average steady state temperature and the normalized time taken to attain steady state are studied. This work is the precursor of experimental research on the anisotropy in thermal conductivity of shape memory alloys, of which virtually none exists in the open literature today.     

Characterization of phase transformation behavior in electrolytically produced indium-thallium shape memory alloy films

Indium-thallium alloys in the range 15–38 at. % Tl were electrodeposited from a sulfate electrolyte by using pulsed current. The relationship between composition and phase structure of the alloys deposited was investigated with regard to the shape memory effect. To date there have been no reported experimental studies describing the electrodeposition of In---Tl alloys capable of undergoing such transformations. Not only did the deposited alloys exhibit the shape memory effect, but the composition-phase relationship exhibited was shown to deviate from that reported for alloys in an equilibrium condition. The results show that the temperature and the temperature interval for the transformation of the electrodeposited alloys are substantially different from those of the thermally prepared alloys. The shape recovery for the electrodeposited alloys was also confirmed.     

成分对多晶Ni-Mn-Ga磁性记忆合金相变行为的影响

研究了Ni50Mn25+xGa25-x和Ni50Mn29Ga21-xTbx两种成分系列磁性记忆合金的相变行为.保持Ni含量不变,增加Mn,降低Ga含量会使马氏体相变温度明显提高,同时相变滞后温区减小,居里温度基本不变.如果添加稀土元素铽、相变温度继续升高,居里温度仍然不变,材料继续保持强的铁磁性及热弹性马氏体相变的特征.     

Burst character of thermoelastic shape memory deformation in ferromagnetic Ni-Fe-Ga-Co alloy

The shape memory deformation in single crystals of Ni₄₉Fe₂₇Ga₁₈Co₆ alloy under uniaxial compression conditions has been studied. It was found that samples in the martensite state deformed in the [110] and [100] directions exhibit strongly anisotropic properties. Despite virtually equal residual strain and the work of deformation in the two cases, the subsequent behavior of samples during shape recovery on heating at a constant rate is principally different. In the former case, the shape memory deformation exhibits a burst character and takes place at a temperature that is significantly higher than the martensite transformation temperature in the initial (undeformed) crystal. In contrast, the shape recovery in the latter case proceeds smoothly at a temperature that is only slightly higher than the martensite transformation temperature in the initial crystal. Due to the burst character of shape memory deformation, the crystal standing on a solid base acquires a velocity above 20 m/s. The results are quantitatively analyzed in terms of the theory of diffuse phase transitions.     

约束态相变中TiNi形状记忆合金的马氏体自拉伸过程

研究了约束态加热过程中产生的回复力对TiNi形状记忆合金剩余马氏体的影响．结果表明，产生的回复力使剩余马氏体发生了塑性变形，使逆相变温度升高，剩余马氏体分数和其逆相变温度之间存在特定的函数关系．但是，外部约束条件的变化对马氏体的自拉伸过程所造成的剩余马氏体分数与其逆相变温度之间的关系影响很小．     

Nanoscale magneto-structural coupling in as-deposited and freestanding single-crystalline Fe7Pd3 ferromagnetic shape memory alloy thin films

Ferromagnetic shape memory alloys are characterized by strong magneto-mechanical coupling occurring at the atomic scale causing large magnetically inducible strains at the macroscopic level. Employing combined atomic and magnetic force microscopy studies at variable temperature, we systematically explore the relation between the magnetic domain pattern and the underlying structure for as-deposited and freestanding single-crystalline Fe₇Pd₃ thin films across the martensite–austenite transition. We find experimental evidence that magnetic domain appearance is strongly affected by the presence and absence of nanotwinning. While the martensite–austenite transition upon temperature variation of as-deposited films is clearly reflected in topography by the presence and absence of a characteristic surface corrugation pattern, the magnetic domain pattern is hardly affected. These findings are discussed considering the impact of significant thermal stresses arising in the austenite phase. Freestanding martensitic films reveal a hierarchical structure of micro- and nanotwinning. The associated domain organization appears more complex, since the dominance of magnetic energy contributors alters within this length scale regime.     

Phase transformation in NiTiHf shape memory alloy thin films

In the present study, amorphous NiTiHf thin films with different compositions were deposited on silicon wafers by using D.C. magnetron sputtering. Crystallization and martensitic transformation characteristics were studied. Crystallization temperatures and activation energy increased with increasing Hf content. The addition of Hf caused larger atomic size mismatch and stronger interactions among constituent elements, thus, increasing the thermal stability of amorphous thin films. With increasing annealing temperature or Hf content, martensitic transformation temperature increased. The results imply that NiTiHf thin films may be used as the potential candidates for high temperature applications in microactuators.